Sea mines over a long history have assumed many forms, but they are generally categorized as "controlled" mines, actuated from a remote source when a target is detected, or "automatic" mines, which are planted unattended and left to their own devices to detonate upon proximity of a target. Automatic mines are subdivided into "contact" mines, which require the target to actually strike it, and "influence" mines, which merely require the target to draw sufficiently near for initiating detonation.
While crude, contact mines have been effective for many years. Still, when navies of the world sought to increase mine capability, they turned to influence mines which may be detonated by some situation causing a local disturbance or change.
Many types of "single" influence sensors for sea mines have been developed, one type of which is the magnetic sensor. It detects a vessel upon its perturbations of the earth's magnetic field. Transducers employing the principle of piezoelectricity to emit electrical signals responsive to acoustic waves have been used in influence sensors for sea mines. More recently, geophones have been employed in bottom mines to sense vibrations of a passing vessel. This type may involve permanent magnetics and a moving coil to generate a signal.
Pressure sensors for mines were developed by Germany in World War II. Their "oyster" mine used this type of sensor. It employed the principle of an air-filled rubber bag which at depth expanded upon exposure to lower water pressures, caused by water flow created by a ship passing in the vicinity, to allow electrical contacts to complete a circuit to explode the mine. While quite sensitive, this type sensor had a limited working depth. This is one of the shortcomings addressed in the present invention.
It is now recognized that the influence signature of a target is a complex phenomenon involving a number of influences including pressure, magnetics, acoustics, and others. Thus, to provide the maximum amount of discriminatory capability for a mine, several sensor output signals must be processed together to yield a single composite "fire" signal based on the simultaneous inter-related behavior of all. Furthermore, with plural influence inputs required to fire a mine there is less chance of inadvertent firing or interference by opposing forces to destroy the mine or render it harmless.
The sensor is adapted to automatically equalize itself to the pressure at ambient depth in which the sea mine is laid, thereby allowing use in practically any waters. It detects variations in pressure from ambient with equal sensitivity regardless of its initial ambient pressure (water depth). It further filters out transient pressure variations which are usually of higher frequency or shorter duration then those caused by a passing ship.